This invention relates to applications of photodetectors and particularly to accurate conversion of detected optical information to electrical signals.
Modulated light sources are used in a variety of applications involving the detection of light reflected from or transmitted through an object, to determine certain properties or characteristics of the object. By way of example, a modulated near infrared light source may be used in connection with the determination of concentrations of various substances, or analytes, in the bloodstream of animals.
A common method of providing a modulated optical signal is to provide a continuous optical source and periodically interrupting the light from the continuous optical source with a chopper. The chopper may be, for example, a rotating segmented disk driven by an electric motor.
The use of a modulated optical source aids in distinguishing, in signal processing, the signal resulting from light from the source from noise and other signals. The signal is typically received at a photodetector, which converts all the optical information within a certain frequency range to an electrical signal. An electrical signal from a photo detector typically includes a large dark signal, a smaller optical-background signal, and a signal due to the source, which is smaller than both the dark signal and the optical-background signal. The use of a modulated optical source permits the use of signal processing techniques that can accurately identify the contribution to the electrical signal of the optical signal of interest. In addition, electronic techniques for processing modulated signals, which are AC signals, are more precise and accurate than techniques for processing unmodulated, or DC signals. It is well known that DC signals suffer from higher noise and larger drift than AC signals.
In the detection of optical signals in the near infrared range, in wavelengths from about 1,000 nanometers to about 2,500 nanometers, lead-sulfide (PbS) detectors are typically used. Within this frequency range, the frequency response is not flat. Alternatively stated, the ratio of the amplitude of the electrical signal output by the detector to the magnitude of the light received by the detector varies with the frequency of the light received.
As a result of the dependency of the response on frequency, any change in modulation frequency, or frequency jitter, causes a change in response, and thus an apparent change in the magnitude of the optical signal. In practice, the typical electro-mechanical system employed to produce modulation is subject to frequency jitter.
It is consequently an object of the invention to provide a method for converting a modulated optical signal to an electrical signal, while providing a flat frequency response.
A further object of the invention is providing a system for obtaining an electrical signal including apparatus for producing a modulated optical signal subject to frequency jitter and an apparatus for converting received signals to electrical signals with flat frequency response.
Additional objects and advantages of the invention will become apparent from the detailed description of the preferred embodiment which follows.